Seismic Response Evaluation of the Control Element Drive Mechanism with a Lateral Seismic Support

A Control Element Drive Mechanism (CEDM) of a pressurized water reactor is an electromechanical device to position the Control Element Assembly (CEA) in the core. The CEDM is vertically mounted on the reactor vessel head like a cantilever so that it is vulnerable to lateral deflection under earthquake excitation. Major safety functions of the CEDM are to sustain the pressure boundary and enable the CEA to fully drop in the core within the acceptable time during earthquake excitation. The acceptance of CEA scram is evaluated for each supporting height as the CEDM's deflection is changed depending on the supporting height. Based on the changes of the seismic responses, desirable installation heights of the seismic support are recommended. The seismic responses for the supported CEDMs have been analyzed with varying the support heights, and assessed by comparing with the loads of the free standing CEDM and scram criteria. The seismic support is capable of reducing the seismic loads only when the support is located at certain height. The guidelines are established to evaluate the deformation of the CEDM against the tested scram criteria. It is confirmed that the CEDM deformation in any supporting conditions satisfies the acceptance criteria.